332 STANDARD ELECTRICAL DICTIONARY.M.(a) Symbol of gaseous pressure equal to one-millionth of an atmosphere.(b) The Greek m, µ, is used as the symbol of magnetic permeability.333 STANDARD ELECTRICAL DICTIONARY.Machine, Cylinder Electric.A frictional electric machine whose rotating glass is in the shape of acylinder instead of a disc as in the more recent machines.Fig. 217. PLATE FRICTIONAL ELECTRIC MACHINE.Machine, Frictional Electric.An apparatus for development of high tension electricity by contactaction, brought about by friction.It consists of a plate or cylinder of glass mounted on insulatingstandards and provided with a handle for turning it. One or morecushions of leather are held on an insulated support, so as to rubagainst the plate or cylinder as it is turned. A metal comb or combs areheld on another insulating support so as to be nearly in contact withthe surface of the glass plate at a point as far removed as possiblefrom the rubbers. The combs are attached to a brass ball or round-endedcylinder, which is termed the prime conductor.In use either the prime conductor or cushions are connected by a chainor otherwise with the earth. Assume it to be the cushions. As themachine is worked by turning the plate, the glass and cushion being incontact develop opposite electricities. The glass is charged withpositive electricity, and as it turns carries it off and as it reachesthe prime conductor by induction and conduction robs it of its negativeelectricity. Meanwhile the cushions negatively excited deliver theircharge to the earth. The action thus goes on, the prime conductor beingcharged with positive electricity.334 STANDARD ELECTRICAL DICTIONARY.If the prime conductor is connected to the earth and the cushions areleft insulated, negative electricity can be collected from the cushions.In some machines both prime conductor and cushions are kept insulatedand without ground contact. Electrodes connecting with each are broughtwith their ends close enough to maintain a sparking discharge.Machine Influence.A static electric machine working by induction to build up charges ofopposite nature on two separate prime conductors. In general they arebased on the principle of the electrophorous. Work is done by theoperator turning the handle. This rotates a disc and draws excited partsof it away from their bound charges. This represents a resistance tomechanical motion. The work absorbed in overcoming this mechanicalresistance appears as electric energy. There are various types ofinfluence machines, the Holtz, Toeppler-Holtz and Wimshurst being themost used. The electrophorous, q. v., is a type of influence machine.Machine, Holtz Influence.A static electric machine. It includes two plates, one of which israpidly rotated in front of the other. Two armatures of paper aresecured to the back of the stationary plate at opposite ends of adiameter. To start it one of these is charged with electricity. Thischarge by induction acts through the two thicknesses of glass upon ametal bar carrying combs, which lies in front of the further side of themovable plate. The points opposite the armature repel electrified air,which strikes the movable disc and charges it. A second rod with comb atthe opposite end of the same diameter acts in the reverse way. Thusopposite sections of the disc are oppositely charged and the combs withthem. By induction these portions of the disc react upon the twoarmatures. The opposite electricities escape from the armatures by papertongues which are attached thereto and press against the back of themovable plate. As the plate rotates the opposite electricities on itsface neutralize the electricity repelled from the combs. The charges onthe back strengthen the charges of the armatures and brass combs. Thusthe machine builds up, and eventually a discharge of sparks takes placefrom the poles of the brass combs.335 STANDARD ELECTRICAL DICTIONARY.Machine, Toeppler-Holtz.A modification of the Holtz machine. The priming charge of the armaturesis produced by friction of metallic brushes against metallic buttons onthe face of the rotating plate. (See Machine, Holtz.)Machine, Wimshurst.A form of static influence machine. It consists of two plates of glass,on which radial sectors of tinfoil are pasted. Both plates are rotatedin opposite directions. The sectors of the two plates react one upon theother, and electric charges of opposite sign accumulate on the oppositesides of the plates and are collected therefrom by collecting combs.Mack.A name, derived from Maxwell, and suggested for the unit of inductance.It is due to Oliver Heaviside, but has never been adopted. (See Henry.)Magne-Crystallic Action.The action of a supposed force of the same name, proposed by Faraday. Itrelates to the different action of a magnetic field upon crystallinebodies, according to the position of their axes of crystallization. Aneedle of tourmaline, normally paramagnetic, if poised with its axishorizontal, is diamagnetic. Bismuth illustrates the same phenomenon. Thesubject is obscure. Faraday thought that he saw in it the action of aspecific force.Magnet.A body which tends when suspended by its centre of gravity to lay itselfin a definite direction, and to place a definite line within it, itsmagnetic axis, q. v., in a definite direction, which, roughly speaking,lies north and south. The same bodies have the power of attracting iron(Daniell), also nickel and cobalt.Magnets are substances which possess the power of attracting iron.(Ganot.)[Transcriber's note: Edward Purcell and others have explained magneticand electromagnetic phenomenon as relativistic effects related toelectrostatic attraction. Magnetism is caused by Lorentz contraction ofspace along the direction of a current. Electromagnetic waves are causedby charge acceleration and the resulting disturbance of theelectrostatic field. (Electricity and Magnetism: Berkeley PhysicsCourse Volume 2, 1960)]Magnet, Anomalous.A magnet possessing more than the normal number (two) of poles. If twostraight magnets are placed end to end with their south poles injuxtaposition the compound bar will seem to possess three poles, one ateach end and one in the middle. The apparent pole in the middle isreally made up of two consequent poles, q. v. It sometimes happens thatwhen a single long thin bar is magnetized consequent poles are produced,although such magnet is in one piece. This may be accidental, as in suchcase it is quite hard to avoid anomalous poles, or, as in the fieldmagnets of some forms of dynamos, anomalous poles may be purposelyproduced.Magnet, Artificial.A magnet formed artificially by any method of magnetization (seeMagnetism) applicable to permanent magnets, electro-magnets andsolenoids. It expresses the distinction from the natural magnets orlodestone, q. v. It is made of steel in practice magnetized by some ofthe methods described under Magnetization.336 STANDARD ELECTRICAL DICTIONARY.Magnet, Axial.A straight-solenoid with axial core.Magnet, Bar.A bar magnet is one in the shape of a bar, i. c., straight with parallelsides and considerably longer than wide or deep.Magnet, Bell-shaped.A form of permanent magnet used in some galvanometers. In shape it is athick-sided cylindrical box with two slots cut out of opposite sides, soas to make it represent a horseshoe magnet. Its shape enables it to besurrounded closely by a mass of copper, for damping its motion, torender the instrument dead-beat. Such a magnet is used in Siemens &Halske's galvanometer.Magnet Coil.A coil to be thrust over an iron core, to make an electro-magnet. Theyare often wound upon paper or wooden bobbins or spools, so as to beremovable from the core if desired.Magnet, Compensating.(a) A magnet fastened near a compass on an iron or steel ship tocompensate the action of the metal of the ship upon the magnetic needle.The ship itself always has some polarity and this is neutralized by oneor more compensating magnets.(b) See below.Magnet, Controlling.A magnet attached to a galvanometer by which the directive tendency ofits magnetic needle is adjusted. In the reflecting galvanometer it oftenis a slightly curved magnet carried by a vertical brass spindle risingfrom the center of the instrument, and which magnet may be slid up anddown on the spindle to regulate or adjust its action.Synonym--Compensating Magnet.Magnet, Compound.A permanent magnet, built up of a number of magnets. Small bars can bemore strongly magnetized than large. Hence a compound magnet may be mademore powerful than a simple one.Magnet Core.The iron bar or other mass of iron around which insulated wire is woundfor the production of an electro-magnet. The shapes vary greatly,especially for field magnets of dynamos and motors. For these they areusually made of cast iron, although wrought iron is preferable from thepoint of view of permeability.Magnet, Damping.A damping magnet is one used for bringing an oscillating body to rest.The body may be a metallic disc or needle, and the action of the magnetdepends on its lines of force which it establishes, so that the body hasto cut them, and hence has its motion resisted.337 STANDARD ELECTRICAL DICTIONARY.Magnet, Deflection of.The change of position of a magnet from the plane of the earth'smeridian in which it normally is at rest into another position at someangle thereto, by the effect of an artificial magnetic field, as thedeflection of a galvanometer needle.Magnet, Electro-.A magnet consisting of a bar of iron, bundle of iron wires, iron tube orsome equivalent, around which a coil of insulated wire is wound. Suchcombination becomes polarized when a current is passed through it and isan active magnet. On the cessation of the current its magnetism in partor almost completely disappears. (See Electro-magnet.)Magnet, Equator of.In a magnet the locus of points of no attractive power and of nopolarity. In a symmetrical, evenly polarized magnet it is the imaginaryline girdling the centre. The terms Neutral Point or Neutral Line havedisplaced it.Synonyms--Neutral Line--Neutral Point.Magnet, Field.A magnet, generally an electro-magnet, used to produce the field in adynamo or motor.Magnet, Haarlem.Celebrated magnets made in Haarlem, Holland. Logeman, Van Wetteren,Funckler and Van der Willigen were the makers who gave the celebrity tothe magnets. They were generally horseshoe magnets, and would carryabout twenty times their own weight.Magnet, Horseshoe.A magnet of U shape--properly one with the poles brought a little closertogether than the rest of the limbs. For direct lifting and attractiveeffects it is the most generally adopted type. Its advantage as regardslifting effect is due to small reluctance, q. v., offered by a completeiron circuit, such as the armature and magnet together produce. As theterm is now used it is applied to any U shaped magnet.Fig. 218. JOULE'S ELECTRO-MAGNET.Magnet, Joule's Electro.An electro-magnet of the shape of a cylinder with a longitudinal segmentcut-off. It is wound with wire as shown. The segment cut-off is a pieceof the same shape as the armature. It is of high power.338 STANDARD ELECTRICAL DICTIONARY.Magnetic Adherence.The tendency of a mass of iron to adhere to the poles of a magnet. It isbest figured as due to the virtual shortening of lines of force, as themore permeable iron gives a better path for them than the air canafford, and consequently a virtually shorter one.Magnetic Attraction and Repulsion.The attraction of a magnet for iron, steel, nickel and cobalt and ofunlike poles of magnets for each other. It is identical withelectro-magnetic attraction, q.v. (Also see Electro-magnetism.)Magnetic Attraction and Repulsion, Coulomb's Law of.Magnetic attraction and repulsion are inversely as the square of thedistance. (Ganot.)While theoretically true in the case of isolated poles, in practise itdoes not generally apply on account of the large diameter and relativeshortness of magnets.Magnetic Axis.The line connecting the poles of a magnet. It does not generallycoincide exactly with any symmetrical axis of figure. In such cases anerror is introduced into the indications of the needle which must bedetermined and allowed for in compasses. To determine it with a magneticneedle the suspension cup is made removable, so that the needle can bereversed. Readings are taken with one side of the needle and then withthe other side of the needle up, and the average corresponds with theposition of the magnetic axis in both positions of the needle.Magnetic Azimuth.The angle, measured on a horizontal circle, between the magneticmeridian and a great circle of the earth passing through the observerand any observed body. It is the astronomical azimuth of a body referredto the magnetic meridian and therefore subject to the variation of thecompass. The angle is the magnetic azimuth of the observed body.Magnetic Battery.A name for a compound permanent magnet; one made up by bolting orclamping together, or to single soft iron pole pieces, a number ofsingle permanent magnets. There are a number of forms of compoundmagnets. In making them care has to be taken to have them of evenstrength. It is also well to have them slightly separated. The object ofboth these precautions is to prevent a stronger element or magnet fromdepolarizing its neighbor.Synonym--Compound Magnet.Magnetic Bridge.An apparatus for testing the relative permeability of iron. It consistsof a rectangular system of iron cores. Three of the sides are wound withwire as shown. The other side is built up of double bars, and from thecentre two curved arms rise, as shown in the cut. The arms do not touch.Between them a short magnet is suspended by a filament, which alsocarries a mirror and an index.339 STANDARD ELECTRICAL DICTIONARY.Fig. 219. MAGNETIC BRIDGE.A lamp and scale are provided as in the reflecting galvanometer. Whenadjusted the magnetic needle hangs as shown in the cut, Fig. 219,without any tendency to turn towards either curved pole piece. If alliron parts are symmetrical and of similar metal, a current through thecoils will make no difference. It will work in magnetic opposition uponthe two arms, or, in other words, will maintain both arms at identicalpotential.Fig. 220. POLE PIECES, MAGNETIC NEEDLEAND MIRROR OF MAGNETIC BRIDGE.If there is the least difference in permeability, length or thicknessbetween any of the iron bars the magnetic potential of the two curvedarms will differ, and the magnetic needle will turn one way or theother. In practical use different samples of iron are substituted forthe unwound members of the fourth side of the parallelogram, and theneedle by its motions indicates the permeability.In the cut, Fig. 220, D D are the ends of the curved pole pieces; A thewire carrying the mirror B and magnetic needle N, and E is the indexwhich shows the larger deflections.340 STANDARD ELECTRICAL DICTIONARY.Magnetic Circuit.A magnetic field of force is characterized by the presence of lines offorce, which, while approximately parallel, curve around and tend toform closed curves. The polarity of a field of force is referred to animaginary direction of the lines of force from the north pole throughspace to the south pole, and in the part of the field corresponding tothe body of the magnet, from the south to the north pole. The cutindicates these features. Hence the magnetic field of force is termedthe magnetic circuit, and to it are attributed a species of resistancetermed reluctance, q. v., and the producing cause of the field or linesof force is termed sometimes magneto-motive force, q. v.) correspondingto the electro-motive force. The modern treatment of the magneticcircuit is similar to the application of Ohm's law and the laws ofresistance and conductivity to the electric circuit.Magnetic Circuit, Double.A magnetic circuit which virtually represents two horseshoe magnetsplaced with their like poles in contact. It is used for field magnets,the armatures occupying a place between the consequent poles.Fig. 221. ONE-HALF PORTION OF A DOUBLE MAGNETIC CIRCUIT.Magnetic Concentration of Ores.The concentration of ores or the freeing them from their gangue bymagnetic attraction. It is only applicable to those cases in whicheither the ore itself or the gangue is attracted by the magnet. Itsprincipal application is to the concentration of magnetic iron sands.(See Magnetic Concentration.)Magnetic Concentrator.An apparatus similar to a magnetic separator, q. v., but used toconcentrate magnetic iron sands. By the action of electro-magnets themagnetic iron sand (magnetite) is separated from the sand with which itis mixed.Magnetic Conductivity and Conductance.The first notion of permeance and of the magnetic circuit included theidea of magnetic conductivity, which conducted lines of force urged bymagneto-motive force through a magnetic circuit. The terms are displacedby permeability and permeance.341 STANDARD ELECTRICAL DICTIONARY.Magnetic Continuity.The completeness of a magnetic circuit, as when the armature of ahorseshoe magnet is in contact with both poles. It is an attribute of aparamagnetic substance only and is identical for permanent magnets orfor electro-magnets. An air space intervening between armature andmagnet poles, or a space filled with any diamagnetic substance preventscontinuity, although the lines of force to some extent still find theirway around. The leakage is increased by discontinuity.Magnetic Control.Control of a magnetic needle, magnet, iron index or armature, in agalvanometer, ammeter or voltmeter by a magnetic field; the restitutiveforce being derived from a permanent magnet.Magnetic Couple.The couple of magnetic force which tends to bring the magnetic needleinto the plane of the magnetic meridian. One force is represented by theimaginary pull upon the north pole, and the other by the opposite pullupon the south pole of the needle. The moment of the couple varies froma maximum when the needle is at right angles to the plane of themagnetic meridian to zero when it is in such plane.Magnetic Creeping.Viscous hysteresis; the slow increase of magnetism in a paramagneticbody when exposed to induction.Fig. 222. MAGNETIC CURVES OR FIGURES.Magnetic Curves.The pictorial representation of magnetic lines of force. It is generallyproduced by scattering filings on a sheet of paper or pane of glass heldover a magnet. The filings arrange themselves in characteristic curves.Tapping the paper or pane of glass facilitates the arrangement, orjarring the filings off a smaller magnet, so that they fall polarizedupon the paper, is thought by some to improve the effect. The group ofcurves forms what are termed magnetic figures, q. v.342 STANDARD ELECTRICAL DICTIONARY.Magnetic Declination.The angular deviation of the magnetic needle, causing it to rest at anangle with the true meridian; the variation of the compass. (SeeMagnetic Elements.)Magnetic Density.The intensity of magnetization expressed in lines of force per statedarea of cross-section in a plane at right angles to the lines of force.Magnetic Dip.The inclination from the horizontal assumed by a magnetic needle free tomove in the vertical plane. (See Magnetic Elements.) The angle of dip orinclination is entirely a function of the earth, not of the needle.Magnetic Discontinuity.A break or gap in a magnetic circuit. To make a complete circuit theiron or other core must be continuous. If the armature of a horseshoemagnet is in contact with both poles the continuity is complete. If thearmature is not in contact magnetic continuity gives place todiscontinuity. It is an attribute of a paramagnetic substance only, andis identical for permanent magnets, or for electro-magnets.Magnetic Elements.The qualities of the terrestrial magnetism at any place as expressed inits action upon the magnetic needle. Three data are involved.I. The Declination or Variation.II. The Inclination or Dip.III. The Force or Intensity.I. The Declination is the variation expressed in angular degrees of themagnetic needle from the true north and south, or is the angle which theplane of the magnetic meridian makes with that of the geographicalmeridian. It is expressed as east or west variation according to theposition of the north pole; east when the north pole of the needle is tothe east of the true meridian, and vice versa. Declination is differentfor different places; it is at present west in Europe and Africa, andeast in Asia and the greater part of North and South America. Thedeclination is subject to (a) secular, (b) annual and (c) diurnalvariations. These are classed as regular; others due to magnetic stormsare transitory and are classed as irregular, (a) Secular variations. Thefollowing table shows the secular variations during some three hundredyears at Paris. These changes are termed secular, because they requirecenturies for their completion.
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